Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste, or adhesive, until a subsequent step in which the solder paste is melted or the adhesive is fully cured.
Pick and place machine operation is challenging. Since machine speed corresponds with throughput, the faster the pick and place machine runs, the less costly the manufactured board will be. Additionally, placement accuracy is extremely important. Many electrical components, such as chip capacitors and chip resistors are relatively small and must be accurately placed on equally small placement locations. Other components, while larger, have a significant number of leads or conductors that are spaced from one another at a relatively fine pitch. Such components must also be accurately placed to ensure that each lead is placed upon the proper pad. Thus, not only must the machine operate extremely fast, but it must also place components extremely accurately.
In order to enhance the quality of board manufacture, fully or partially populated boards are generally inspected after the placement operation(s), both before and after solder reflow, to identify components that are improperly placed or missing or any of a variety of errors that may occur. Automatic systems that perform such operation(s) are highly useful because they help identify component placement problems prior to solder reflow. This allows substantially easier rework and/or the identification of defective boards after reflow that are candidates for rework. One example of such a system is sold under the trade designation Model KS Flex available from CyberOptics Corporation of Golden Valley, Minnesota. This system can be used to identify such problems as alignment and rotation errors; missing and flipped components; billboards; tombstones; component defects; incorrect polarity; and wrong components.
Identification of errors pre-reflow provides a number of advantages. Rework is easier; closed-loop manufacturing control is facilitated; and less work in-process exists between error generation and remedy. While such systems provide highly useful inspection, they do consume plant floor-space as well as programming time and maintenance efforts.
One relatively recent attempt to provide the benefits of after-placement inspection located within a pick a place machine itself is disclosed in U.S. Pat. No. 6,317,972 to Asai et al. That reference reports a method for mounting electric components where an image of a mounting location is obtained prior to component placement, and compared with an image of the mounting location after component placement to inspect the placement operation at the component level.
While the disclosure of Asai et al. marks one attempt to employ in-machine component level inspection, there remains much work to be done. For example, the disclosure of Asai et al. teaches acquiring two images before and after the placement of the component to determine placement characteristics of the component. While this approach is useful for determining the absence or presence of a component after placement, there are several important machine characteristics of the placement machine that can cause placement errors of components that this approach does not address.
One major contributing factor to the quality of the component placement is movement and vibration of the workpiece during the placement operation. Such vibration and/or motion of the workpiece can be caused by: the length of the nozzle; the distance of vertical travel of the nozzle during the placement cycle; the rigidity of the workpiece; and the placement of workpiece support.
To increase the viability of component level placement inspection in a pick and place machine, it would be advantageous to remove or minimize the effects of such motion. Reduction of vibration and/or motion effects would allow components to be placed more accurately thereby facilitating the cost effect manufacture or smaller-scale and/or higher density workpieces. Moreover, reduction or elimination of motion effects may allow components to be placed more quickly since the pick and place machine would not require as much time between ceasing relative x-y motion between the nozzle and the workpiece and placing the component.